With an explosive increase in demand for broadband multimedia communication services in such as the Internet and video distribution, the introduction of a long-distance, high-capacity, highly reliable optical fiber communication system is in progress.
In an optical fiber communication system, it is important to cut the installation cost of an optical fiber serving as an optical transmission line and improve the transmission band utilization efficiency per optical fiber. Under the circumstances, digital coherent optical communication technology using a digital optical transceiver is attracting a great deal of attention in recent years and growing in importance.
An optical communication system employs an analog optical transceiver using a hitherto widely applied modulation scheme such as OOK (On-Off Keying). In a digital coherent optical transceiver, even when it includes waveform distortion due to such as wavelength dispersion occurring in a transmission line or transceiver imperfectness, the distortion can be compensated for by performing DSP (digital signal processing) on the transmission or reception side.
Such technology improves the performance of an optical communication device and can achieve such as a low cost, and a high-capacity trunk optical communication system is thus becoming popular.
In particular, by taking advantage of digitization, a multilevel modulation technique such as QPSK (Quadrature Phase Shift Keying) modulation or QAM (Quadrature Amplitude Modulation) modulation that is hard to implement in analog processing becomes applicable. A technique for increasing the number of wavelength multiplexed channels by signal band narrowing using Nyquist filtering also becomes applicable. This improves the transmission band utilization efficiency, thus allowing a higher transmission capacity per optical fiber.
However, with the application of an advanced transmission scheme such as the aforementioned multilevel modulation technique or Nyquist filtering technique, the signal waveform becomes more complex, thus requiring a higher signal accuracy.
Characteristic degradation of an analog front end unit constituting a transceiver such as a transmission light modulator or a coherent detection receiver may therefore more significantly influence its transfer characteristic.
FIG. 10 is a block diagram illustrating the general configuration of a digital coherent optical receiver. The digital coherent optical receiver illustrated in FIG. 10 includes a photoelectric (O/E) conversion unit 101, a reception analog front end unit 102, and a digital demodulation processing unit 105.
The photoelectric (O/E) conversion unit 101 performs coherent detection by mixing an optical input signal and a local oscillator signal (LO light: Local-Oscillator light) together and converts four obtained optical signals (X-polarized I/Q signals and Y-polarized I/Q signals) into analog electric signals.
The reception analog front end unit 102 amplifies and converts the four analog electric signals into digital signals. The reception analog front end unit 102 is configured by including an amplifier 103 and an ADC (Analog-to-Digital Convertor) 104. The amplifier 103 amplifies each electric signal generated by optical/electrical conversion to an amplitude sufficient for signal processing. The analog-to-digital converter 104 converts each amplified analog electric signal into a digital signal. The analog-to-digital converter 104 will be referred to as an A/D converter 104 hereinafter. The digital demodulation processing unit 105 demodulates a signal using, as input, a digital signal corrected in linearity.
Examples in which characteristic degradation of the analog front end unit significantly influences its transfer characteristic, as described above, include the linearity of the reception analog front end unit. Such types of characteristic degradation result in distortion such as a deviation from an ideal constellation position or deformation, thus significantly degrading the transfer characteristic.
FIGS. 11A and 11B are diagrams illustrating specific examples of constellation distortion, in which FIG. 11A illustrates an ideal constellation, and FIG. 11B illustrates a constellation when the linearity has degraded in the input/output transfer characteristic of a reception front end unit. Note that the constellation defines the arrangement of signal points representing a combination of the phases and/or amplitudes of an in-phase channel (I channel) and a quadrature channel (Q channel) in a digital quadrature modulation scheme such as QPSK or 16-QAM.
It is a hitherto common practice to compensate for such linearity degradation of the reception analog front end unit by performing arithmetic processing of a characteristic inverse to the transfer characteristic of the reception analog front end unit in a reception digital signal processing unit, as illustrated in FIG. 12. FIG. 12 is a configuration diagram of the digital optical receiver in the background art, for correcting the linearity.
The digital optical receiver illustrated in FIG. 12 includes a photoelectric (O/E) conversion unit 101, a reception analog front end unit 102, and a digital demodulation processing unit 105, like the digital optical receiver illustrated in FIG. 10. The digital optical receiver illustrated in FIG. 12 further includes a linearity correction unit 106 and a control circuit 107 for the linearity correction unit. The linearity correction unit 106 imparts an arbitrary transfer characteristic to each of four digital signals and corrects the linearity of this digital signal. The control circuit 107 controls the amount of correction (correction parameter) of the linearity correction unit 106.
FIG. 13A illustrates, as an example, the input/output transfer characteristic of an amplifier 103 constituting the reception analog front end unit 102. FIG. 13B illustrates the inverse input/output transfer characteristic of the amplifier 103 constituting the reception analog front end unit 102, for distortion correction. An amplifier implemented as a transistor has a slightly curved input/output transfer characteristic, unlike an ideal characteristic indicated by a broken line, as illustrated in FIG. 13A.
The digital optical receiver in the background art illustrated in FIG. 12 includes a linearity correction unit 106 and a control circuit 107 that controls the linearity correction unit 106, and compensates for the linearity by performing arithmetic processing of a characteristic inverse to the transfer characteristic of the reception analog front end unit. The linearity correction unit 106 illustrated in FIG. 12 corrects the linearity by applying a characteristic inverse to the input/output transfer characteristic of the reception analog front end unit 102, that is, a correction function as illustrated in FIG. 13B. The linearity can be corrected when the input/output transfer characteristic of the reception analog front end unit 102 is static and known because, for example, this characteristic has been measured in advance.
Patent Literature 1 (PTL1) relates to a digital receiver and, more particularly, to control for an analog-to-digital converter that converts an analog electric signal from a photoelectric converter constituting a digital receiver into a digital signal. PTL1 proposes setting a plurality of identification levels used as a determination criterion in A/D conversion, in accordance with an identification level control signal from an identification level adjustment circuit.